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10/26/2012
1
Java Decision Making
and booleans (Java: An Eventful Approach, Ch 4),
Slides Credit: Bruce, Danyluk and Murtagh
CS 120 Lecture 14
26 October 2012
Conditional Statements
• Consider:
if the mouse location is contained
in the rectangle, display message “success”
• Some programming constructs can
choose between blocks of code to execute
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A Vote Counting Example
Complete source code
Code to Update Votes:
// Update votes and display vote counts
public void onMouseClick( Location point ) {
if ( point.getX() < MID_X ) {
countA++;
infoA.setText( "So far there are " + countA +
" vote(s) for A." );
} else {
countB++;
infoB.setText( "So far there are " + countB +
" vote(s) for B." );
}
}
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Syntax of the if Statement
• The condition is evaluated. If true, the bracketed code following if is executed; otherwise, the bracketed code after else is executed
if (condition) {
//do something
} else {
//do something else
}
• The else part can be omitted
if (condition) {
//do something here
}
if if-else
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if Statement and 2D Objects
To check if the mouse is within a 2D object:
public void onMouseClick ( Location point ) {
if ( anObject.contains ( point ) ) {
//do something here
}
}
Comparison Operators
Operator Description
> Greater than
< Less than
== Equal to
<= Less than or equal to
>= Greater than or equal to
!= Not equal to
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• Examples
If a=25, b=30
a<b evaluates to true
a<=b evaluates to true
a==b evaluates to false
a!=b evaluates to true
a>b evaluates to false
a>=b evaluates to false
Since these expressions are evaluated to either true or false,
they are called boolean expressions
The boolean Data Type
• A data type that has only two values: true
or false
• Can be declared with the word boolean
Ex: boolean a=true; //holds the value true
boolean b=c>d; //holds the result of c > d
boolean k; //creates k, which can take
on true or false
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Dragging a Box
//boolean variable to determine whether the box is grabbed
private boolean boxGrabbed;
// Save starting point and whether point was in box
public void onMousePress( Location point ) {
lastPoint = point;
boxGrabbed = box.contains( point );
}
// if mouse is in box, then drag the box
public void onMouseDrag( Location point ) {
if ( boxGrabbed ) {
box.move( point.getX() - lastPoint.getX(),
point.getY() - lastPoint.getY() );
lastPoint = point;
}
}
Complete source code
More Uses of The if else Statement
• Picks one choice among many EX: Converting a score into a letter grade
if ( score >= 90 ) {
gradeDisplay.setText( "The grade is A" );
} else if ( score >= 80 ) {
gradeDisplay.setText( "The grade is B" );
} else if ( score >= 70 ) {
gradeDisplay.setText( "The grade is C" );
} else {
gradeDisplay.setText( "No credit is given");
}
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Combining Multiple Conditionals
• && (and) combines adjoining conditions in a way that the final result will be true only if all are true Ex: a && b && c
is true if a,b,c are true
• || (or) combines adjoining conditions in a way that if any of them is true, the final result will be true
Ex: a || b || c
is true if any of a, b, c, is true
The Craps Example
• A block of code that uses || (or) to determine whether the player wins a game of Craps
if ( roll == 7 || roll == 11 ) { // 7 or 11 wins on first throw
status.setText( "You win!" );
} else if ( roll == 2 || roll == 3 || roll == 12 ) { // 2, 3, or 12 loses
status.setText( "You lose!" );
} else { // Set roll to be the point to be made
status.setText( "Try for your point!" );
point = roll;
…
}
Complete source code
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Nesting
• Suppose we want to decide among several
choices based on several conditions, such as
shown by the table:
• To do this, we use conditionals inside a
conditional. This is called nesting.
Sunny Not sunny
Rich Outdoor Concert Indoor Concert
Not Rich Ultimate Frisbee Watch TV
Nesting Example
if ( sunny ) {
if ( rich ) {
activityDisplay.setText( "outdoor concert" );
} else { // not rich
activityDisplay.setText( "play ultimate" );
}
} else { // not sunny
if ( rich ) {
activityDisplay.setText( "indoor concert" );
} else { // not rich
activityDisplay.setText( "watch TV" );
}
}
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Creating a Game of Craps
By adding an outer conditional, we can effectively determine whether the player wins:
if ( newGame ) { //Starting a new game
if ( roll == 7 || roll == 11 ) { // 7 or 11 wins on first throw
status.setText( "You win!" );
} else if ( roll == 2 || roll == 3 || roll == 12 ) { // 2, 3, or 12 loses
status.setText( "You lose!" );
} else { // Set roll to be the point to be made
status.setText( "Try for your point!" );
point = roll;
newGame = false; // no longer a new game
}
}
Complete source code
Student To Do’s
• HW6 (Three problems)
– Bring your solutions to lab.
– (If no class on Monday, check your email for submission
instructions)
• Read Java: An Eventful Approach
– Ch. 4 (Today)
– Ch. 5-6 (Next week)
• Don’t fall behind! We’re moving quickly through Java topics
because we already know the concepts from Alice!
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